The present invention relates to a certain epoxy resin composition and a coating composition containing the epoxy resin composition.
Conventionally, epoxy resins are used in various fields as they can form a coating film having excellent adhesiveness to metal, retort resistance, chemical resistance, workability and the like. However, in the case of heat-curable epoxy resin coatings, such as can inner coatings, PCM coatings or powder coatings, that requires high performance, there is a problem that a baking furnace and the like are stained by fume generated in baking. Cause of this fume is considered to be volatilization of low molecular weight components or decomposition products contained in amino resins, phenolic resins, resole resins, urea resins, blocked isocyanates or the like used as a curing agent, and blocking agents dissociated, and volatilization of low molecular weight components or their decomposition products contained in epoxy resins. Various materials have conventionally been reported as decomposition products generated by thermal decomposition of epoxy resins. As one of those, a monomer component, such as bisphenols generated from bisphenol type epoxy resins, have recently become a problem as one of components staining a baking furnace or the like.
Therefore, it is desirable to provide an epoxy resin composition that does not stain a baking furnace or the like by suppressing generation of components such as bisphenols generated and volatilized by thermal decomposition of epoxy resins, and a coating composition containing such epoxy resin composition.
An epoxy resin composition comprising at least one epoxy resin a thermal decomposition inhibitor for an epoxy resin in an amount of 0.001 to 5 parts by weight per 100 parts by weight of an epoxy resin.
As a result of extensive investigations on a method for suppressing bisphenols or the like decomposed and generated in baking, inventors of the present invention have found that volatile components generated in thermal decomposition can be suppressed by using a thermal decomposition inhibitor in an epoxy resin composition. We have found that the epoxy resin composition of the invention suppresses generation of volatile components by decomposition in baking and does not stain a baking furnace, without impairing physical properties such as workability, adhesiveness, retort resistance, chemical resistance, etc. The present invention includes the following.
The epoxy resin used in the present invention includes bisphenol type epoxy resins such as bisphenol A, bisphenol F, bisphenol S or halogenated bisphenol A, (bisphenol-based), biphenyl type epoxy resins, ester type epoxy resins such as dibasic acid, alicyclic epoxy resins, and terpenediphenol type epoxy resins. Further, alkyl phenol novolak epoxy resins such as phenol novolak epoxy resins or cresol novolak epoxy resins, novolak epoxy resins of bisphenol A, and the like can also be used. Those epoxy resins may be used in one kind or a combination of two kinds or more.
Particularly preferable epoxy resin is a bisphenol type epoxy resin having an epoxy equivalent of 170 or more, preferably 400 or more, and more preferably 1000 or more. Examples of such an epoxy resin include bisphenol A type epoxy resins (bisphenol A based, where the epoxy resin is derived from bisphenol A by glycidation and optionally modification by advancement and the like), bisphenol F type epoxy resins, and epoxy resins obtained by direct synthesis method or indirect synthesis method of their mixture, wherein those epoxy resins have a bisphenol A content of 10 ppm or less, and preferably 3 ppm.
If the bisphenol A content is not less than 10 ppm, this content itself stains a baking furnace, which is not desirable. Preferably, the epoxy resin contains a component which volatilizes by heating in an amount of 10 ppm or less.
The epoxy resin composition of the present invention contains a thermal decomposition inhibitor. The thermal decomposition inhibitor used herein is generally called an antioxidant.
The thermal decomposition inhibitor used in the present invention includes phosphorus type, sulfur type or phenol type compounds. The phenol type thermal decomposition inhibitor has a large decomposition suppression effect because of its radical capturing function, and is, therefore, particularly preferable.
Specifically, the phosphorus type compounds include tris(nonylphenyl)phosphite and tris(2,4-di-tert-butylphenyl)phosphite. The sulfur type compounds include dilauryl-3,3xe2x80x2-thiodipropionate and distearyldilauryl-3,3xe2x80x2-thiodipropionate. Phenol type compounds include tetrakismethylene-3(3,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl) propionate methane, n-octadecyl-3(3,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate, and 2,2xe2x80x2-methylenebis(3-methyl-6-tert-butylphenol).
Of those, tetrakismethylene-3(3,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate methane and n-octadecyl-3(3,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate are preferable. Those thermal decomposition inhibitors may be used alone or in combination of two kinds or more.
The preferred thermal decomposition inhibitor for an epoxy resin is a phenolic thermal decomposition inhibitor.
The amount of the inhibitor compounded is not particularly limited, but it is used so as to be in the range of 0.001 to 5 parts by weight, preferably 0.005 to 2 parts by weight, and more preferably 0.01 to 1 part by weight, per 100 parts by weight of the epoxy resin. If the compounding amount is not larger than 0.001, decomposed products generated by thermal decomposition can not sufficiently be suppressed, and on the other hand, if the amount is not less than 5 parts by weight, decrease in physical properties is remarkable, which is not desirable.
A method for compounding the thermal decomposition inhibitor is not particularly limited, but it is preferable to add the inhibitor after completion of the reaction of the epoxy resin or after melting the epoxy resin, and then melt mixing at 50 to 250xc2x0 C. for 30 minutes to 5 hours. Further, the inhibitor can also be added in the course of or after completion of the coating production step.
The curing agent used in the epoxy resin coating composition of the present invention is curing agents used in general epoxy resins, such as polyamide amines, epoxy resin amine adducts, aliphatic polyamines, modified polyamines, aromatic amines, tertiary amines, hydrazide, dicyanediamide, imidazole, acid anhydrides, acid-terminated polyester resins, phenolic resins, urea resins, resole resins, amino resins, isocyanates and blocked isocyanates. Those can be used according to the purpose. Of those curing agents, curing agents that require baking at high temperature, such as phenolic resins, urea resins, resole resins, amino resins and blocked isocyanates are preferable.
The curable epoxy resin composition preferably contains a curing agent for an epoxy resin in an amount of 0.01 to 50 parts by weight per 100 parts by weight of the epoxy resin.
If necessary, the epoxy resin coating composition of the present invention can further contain resins such as xylene resin, petroleum resin, acrylic resin, polyester resin, alkyd resin, polyimide resin or epoxy resin, reactive or non-reactive diluents such as monoglycidyl ethers, dioctyl phthalate, benzyl alcohol or coal tar, fillers such as glass fiber, carbon fiber, silica, aluminum hydroxide, titanium dioxide, carbon black or iron oxide, and additives commonly used such as pigment, thickener, thixotropic agent, coloring agent, flow modifier, catalyst, curing accelerator, defoaming agent or various solvents.